In recent years, in the field of automobiles, air vehicles, or the like, to detect obstacles in the moving direction, object detection devices of optical scanning type have been developed and actually used, each of which emits, for example, a laser beam during scanning, receives light reflected from an object, and acquires obstacle information on the basis of a time difference between light emission and light reception.
In addition to the obstacle detection for mobile products as described above, such object detection devices may be used for crime prevention to detect suspicious individuals by being installed under eaves of buildings, may be used for geomorphic investigation or the like to acquire geomorphic information from above by being mounted to helicopters, aircrafts, or the like, or may be used for gas detection to measure the concentration of a gas in the air.
In general, an object detection device of optical scanning type includes a light projecting system which includes a laser diode as a light source and a collimator lens, and a light receiving system which includes a light receiving lens (or a mirror) and a light detection element such as a photodiode, and a reflection mirror which includes a reflection surface disposed between the light projecting system and the light receiving system. In such an object detection device of laser scanning type, the reflection mirror is rotated to scan and project light emitted from the light projecting system, having the merit of, not measuring an object at one point, but two-dimensionally widely measuring the object. Note that, as the light source, an LED or the like may be used in addition to the laser.
As an example, for a laser beam source, a general laser beam scanning method is known to project a laser beam to a mirror or a polygon mirror having a plurality of mirror surfaces, swing the mirror or rotate the polygon mirror, and scan the laser beam.
Patent Literature 1 discloses a configuration in which a rotation mirror has a first reflecting surface and a second reflecting surface to make an angle of 90°, and a beam emitted from a light source in a direction orthogonal to a rotation shaft is reflected twice on the first and second reflecting surfaces for scanning, preventing distortion of a scan line even if the rotation shaft tilts due to a rotational deviation. Furthermore, in Patent Literature 2, a laser radar is disclosed in which a plurality of pairs of first and second mirrors are disposed, and the crossing angles between the first mirrors and the second mirrors of the respective pairs are different from each other, enabling scanning at a plurality of different vertical scanning positions in single rotation.